Abstract: The mudstone section in piedmont structures at the south margin of Junggar Basin is characterized by high clay content and fragility,which caused drill pipe stuck,lost circulation,borehole instability,slow penetration rate and low drilling efficiency.In view of these problems,the topic of increasing penetration rate has been carried out.Proceeding from stress characteristics of the region that belongs to thrust nappe fault and fold,mechanical properties of mudstone was analyzed in detail,including borehole stability,formation creep rate and affecting factors of rheological formation,sidewall rock collapsing mechanism,mineral composition and physical and chemical characteristics of the formation.A technique of raising penetration rate in the piedmont belt was put forward,which had been used in exploration Well Xihu 1.The application showed that the strong inhibitory and plugging ability of PRT organic salt drilling fluid system,could effectively reduce borehole instability caused by water absorption swelling,and prevent the loss circulation,combined PDM with PDC bit could improve penetration rate significantly,the average penetration rate had reached to 3.00 m/h.Compared with the offset Well Xi 5,the depth of Well Xihu 1 is deeper than that of former by 20.5%,but the downhole trouble is far less than Well Xi 5,drilling cycle time shortened by 11.9%.It has proved that the proposed the technique of raising penetration rate could be effectively applied in the piedmont belt.